London can be mean, moody and foul-mannered. Sometimes it is a dispiriting place to be: too artsy, too class-ridden, absurdly obsessed with the activities of its resident cast of snobs, fops and clots.

But London can also be one of the liveliest, most stimulating and hedonistic of world capitals, especially now that global warming is turning it into a version of Rome-on-the-Thames: glamorous, glittery, sexy, dangerous. (It has always been decadent.)

London is not a beautiful or an imposing city. On those scores it trails far behind Paris, for example, and probably always will. Its transport systems are a mess. It is not always as friendly as it would have you believe. And it is too easily pleased with itself. Lately, London's narcissism has convinced it that it is a global epicentre of creativity - food, fashion and film-making especially - even though the evidence for such a belief is slim.

In spite of all that, London is doing fine, and is held in great affection by regular visitors, few of whom, I wager, would disagree with the proposition that it is among the world's 10 (or 15) greatest cities.

I have lived and worked in London for 25 years, but have only recently started to make sense of it, or to appreciate it fully. I have come to believe that the key to London is not to try too hard with it, but to let it reveal itself in its own time and manner.

Visitors hoping to enjoy London might do well to follow a few simple rules. First, stick to the centre. Everything you could possibly want is in central London. Stay away from the suburbs. Above all, do not venture south of the river. The exceptions to this are the South Bank arts complex, which includes the National Theatre and Festival Hall, and the Globe Theatre. Both of these attractions are only just across the river. Otherwise, south London is appalling. I should know: I lived there for 12 years. Sometimes I used to weep.

In addition to not straying south of the Thames, my advice is: do not go further east than the Tower of London, further north than Marble Arch or further west than Kensington Palace and Notting Hill.

Second, do not use a car. Driving in London is impossible. In turn, the Tube trains and the buses are the despair of commuters. There are times when London's rush-hour seems near-continuous, but the Tube is generally okay between 10 a.m. and 4.30 p.m., and in mid-to-late evening.

I would even use taxis as infrequently as possible. Some people claim to be enamoured of London's cabbies, but I have always found them irritating and ignorant. Some time ago I had lunch with Marco Pierre White, London's most celebrated three-star chef. I discovered that White shared my hatred of London's taxi drivers.

I said that most of them were fascists. "Yah," agreed White. "You've got it: fascists." The master-chef said he did not own a flashy car. "Don't actually own a car (at all) 'cause I don't even drive. But my girlfriend's got an off-roader, the biggest you can buy, which I'm fitting out with bumper-guards and really major spotlights in case any taxi drivers want to take us on."

Third, stay in the best hotel you can afford, preferably in Mayfair or St James's. The sort of hotel I have in mind is the Metropolitan, on Park Lane, on the western edge of Mayfair. The Metropolitan is one of London's best so-called designer hotels, the haunt of a sophisticated, post-ironic, CK crowd. If you feel uncomfortable in places such as that, there are plenty of traditional five-star hotels, equally uncheap, elsewhere in Mayfair and St James's.

Fourth, spend only a short time visiting London's best-known tourist sights: St Paul's Cathedral, the Tower of London, Westminster Abbey, and so on. On the other hand, while London is not the capital of all-round creativity that it imagines itself to be, it is a world-beater in terms of the performing arts - theatre, music, dance - and has outstanding art galleries.

The best way to let London reveal itself to you is to stroll aimlessly about, making use, whenever possible, of the city's open spaces, particularly the central parks - Hyde Park, St James's Park, Green Park, and the like - which are one of London's most distinguishing features. Whenever you can, abandon the busy streets and nip into one of these parks. With the aid of a map, you can use them as stepping stones along your routes.

Only by walking can you revel in another of London's best features, its atmosphere - not the stuff you breathe, which is getting extremely mucky, but the spirit of the place, its mood of liberty and libertinism. It is important to have both of those together: in London they go hand in glove.

Recently, I bought a copy of Andrew Duncan's book Secret London to see what it had to offer. Duncan is said to be an expert on London's history and geography, and his book claims to take the lid off hidden London, describing 20 miles of little-known walks and 30 unusual places to visit, including roof gardens, secret passages and lost rivers.

In reality, Secret London is not especially revealing. But it has a lot of charm, its main contribution to an understanding of London being the way it forsakes the obvious and well-known. Instead, it hustles you down alleyways and unmarked lanes whenever possible.

Alleyways and unmarked lanes are the surest way to get to know a city's texture, and I was not disappointed by the Secret London walk on which I embarked.

Because of lameness caused by a tennis injury, I chose the easiest walk in the book: a three-and-a-half mile, two-and-a-half hour stroll from Baker Street Tube station to the Thames embankment on the edge of Pimlico. Duncan calls this his Tyburn River walk: one of three perambulations in Secret London that follow the course of what he describes as London's most significant "lost" tributaries of the Thames.

Of the Tyburn itself, however, I saw not so much as a bucketful. This is because all the former north-bank, central London tributaries of the Thames, with the exception of the Lea, have been covered over and turned into storm sewers. Many clues to their former existence are nevertheless still visible above ground, says Duncan.

The Tyburn is said to rise (or to have risen) on Haverstock Hill, near Hampstead; to flow down through Swiss Cottage; enter Regent's Park; then starts its journey through the West End in the Baker Street area.

So I started down the left-hand side of Baker Street, as instructed, and turned left into Paddington Street. Moments later my route took me into Paddington Street Gardens, one of those little-known parks with which London is studded. The gardens were formed in the 18th century as an additional burial ground - the site is still consecrated - for the old St Marylebone Parish Church. There is no visible sign of them, but there are still around 80,000 graves in the gardens.

Limping slowly through the posh residential area of Mayfair - still allegedly following the ups, downs, twists and turns of the Tyburn valley - I ducked down several alleyways, as the route demanded, and stopped for a sandwich at a small cafe in a part of Mayfair I hadn't known existed.

Ten minutes later, I picked my way through the part of Mayfair known as Shepherd Market. The original market from which this enclave of narrow streets, pubs and restaurants takes its name must have grown up on the banks of the Tyburn River, says Secret London.

Then I crossed Piccadilly and entered Green Park. Here, a long time ago, the open Tyburn disappeared into the marshy ground of the Thames flood plain. Now, safely contained in its brick-lined tunnel, the Tyburn continues on its way beneath Buckingham Palace to its junction with the Thames, about one and one-third miles away.

So I pressed on: across Constitution Hill, along the front of Buckingham Palace (I looked, but there were no pale Windsor faces at the windows), across Victoria Street, over Vauxhall Bridge Road and into Pimlico, heading for the point where the remnant Tyburn is alleged to join the Thames.

Beneath Tyburn House and Rio Cottage, says Duncan, there is a semi-circular opening in the Thames wall about 10 feet high and 15 feet across. Set back about 20 feet inside it is a heavy iron sluice gate installed in 1832. If you go round the corner into Crown Reach Riverside Walk and look over the parapet, you can see the Tyburn's outflow channel.

So I did. The tide was out, the Thames at its lowest ebb. A police launch was travelling upstream, hunting desperadoes. Gulls flapped listlessly along the riverbank.

There was no one about. I leaned over the parapet, but there was nothing at all to see. Not a trace of the old Tyburn. Not a bucketful. Not a spoonful. Not a single watery molecule. A ghost river indeed.

Secret London is published by New Holland (Publishers), and costs £9.99 (sterling).